The present invention is concerned with a hydraulic braking and steering brake system, especially for use with farm tractors, comprising two master cylinders operable singly or jointly by a brake pedal. Each master cylinder has primary and a secondary piston operating in a bore. The secondary piston together with the primary piston and the bore form the primary pressure chamber and, with the closed end of the bore, form the secondary pressure chamber. When the primary and secondary pressure chambers are in the release position via channels closeable upon actuation of the primary and secondary pistons, closing elements are connected to an equalizing chamber. The secondary pressure chambers are interconnected through an equalizing line and the primary pressure chambers are connected to steer-brakeable brakes on wheels at opposite sides of the vehicle. A valve arrangement respectively interconnects the two primary pressure chambers in the event that the two master cylinders are actuated simultaneously, separating them in the event that only one of the master cylinders is actuated.
A hydraulic brake and steering brake system of this type has already been described in the prior German Patent Application No.(P33 05 856). In the braking system according to the reference are two tandem master cylinders that can be operated singly or jointly, with each of the tandem master cylinders being provided with a primary pressure chamber and a secondary pressure chamber. Connected to the primary pressure chamber of each master cylinder is a wheel brake of a steer-brakeable vehicle axle, preferably the rear axle of a vehicle. The secondary pressure chambers are interconnected via an equalizing line in each operational condition of the brake system. Provided on the equalizing line is a change-over valve operable by the pressure of the secondary pressure chambers. This valve, in the release and steer-brake position through its resting position, hydraulically separates the steer-braked wheel brakes. In the brake position, through actuation thereof, the valve interconnects the same steer-braked wheel brakes.
Actuation of the change-over valve will be performed only in the braking position as the two secondary pistons are then actuated at the same time, with the secondary pressure chambers being sealed in pressure-tight manner after traversing the channels and breather bores, respectively. Pressure can now build up in the secondary pressure chambers and in the equalizing line, that will act upon the actuating piston of the change-over valve.
As in the steer-brake position, only one master cylinder will be operated. The secondary pressure chamber of that master cylinder will be sealed in pressure-tight manner, however, no further pressure can build up in the secondary pressure chamber since, upon a further displacement of the piston, the volume of the displaced brake fluid will be displaced via the equalizing line, the secondary pressure chamber and the breather bore of the non-operated master cylinder toward the equalizing reservoir.
In the valve piston of the change-over valve there is a channel connected to the equalizing line, in which channel is disposed a valve closeable in the direction of the wheel cylinder pressure chambers of the front wheel brakes.
Valve closure is required in order to permit, on the one hand, in the braking position, a displacement of the change-over valve piston and, on the other hand, in the release position thereof, a pressure balance from the reservoir to the front wheel brakes.
The disadvantages of the reference braking and steering brake assembly lie in the relatively complex construction of the change-over valve arrangement and the fact that in each vehicle braking operation occurrence of which is far more frequent than a steering brake operation, the change-over valve will have to be hydraulically activated., Accordingly, in each braking operation, due to an increase in volume through displacement of the change-over valve, extended braking pedal paths will have to be accepted. As the change-over valve, as a result of a permanent actuation is subject to increased wear, the frequency of failure of a braking system is liable to increase thereby.